(1) Field of Invention
The present invention relates generally to means for protecting electrical circuit components, and more particularly to means for protecting such components from damaging surge effects caused by hostile, external, high intensity, transient electromagnetic fields.
(2) Summary of the Prior Art
Designers of high voltage, laser, and similar equipment have understood for many years that while commonly available circuit components such as resistors, inductors, rectifiers, and even solid state elements are substantially failure free devices under DC operating conditions, the necessity of locating such components in regions in which high intensity transient electromagnetic fields are possible, or are prone to occur, renders such components vulnerable to failure due to the induced surge effects caused by such transient fields. Accordingly, it is well recognized that circuit components which are vulnerable to external transient fields must either be strong enough to withstand an induced surge or be protected therefrom. The use of components having electrical ratings which indicate an ability to withstand expected surge levels is unattractive due to the extreme cost differential between such special components and those more commonly available.
The primary emphasis for the designer has thus been to protect the components in the most efficient manner possible. In theory this goal can be accomplished either by isolating the component in a substantially uniform constant field unaffected by the transient or by adding components to the circuit whose sole function is to draw off surges before they can destroy an important component. In practice, however, the latter is the alternative which has been consistently chosen. By using well known filtering techniques, such as connecting a capacitor in parallel with the component to be protected, designers have been able, at less cost than that of special components, to provide at least moderate levels of protection for components subjected to low level surges. It must be recognized, however, that this level of protection is imperfect especially with respect to surges caused by high intensity transients, and is expensive in terms of added circuit complexity, and weight and space considerations as well as in terms of money.
The former of the above-stated alternatives has heretofore not been given serious consideration for reasons which will become apparent below.
The most common means for creating a region of substantially uniform electric field is the common parallel plate capacitor which is known to have such a field in the region between the central portions of the respective plates. The problem is that localized field stresses and nonuniformities particularly at the edges of parallel plate capacitors set the break down value of the whole assembly at a significantly lower level than would be theoretically expected from a consideration of the dielectric strength of the material between the plates alone. This in turn means that capacitance, potential across the plates, and electric field between the plates are all limited. Accordingly, a component located in the region between the central portions of adjacent plates would be substantially isolated from transient field induced surge effects only so long as the break down value of the assembly was not reached, this value being significantly lower than the generally expected to be induced by the high intensity transients common in high voltage equipment.
Certain workers in the art, specifically C. S. Franklin in U.S. Pat. No. 1,628,659, F. S. Smith in U.S. Pat. No. 1,836,195 and R. J. Van DeGraaff in U.S. Pat. No. 3,296,826, have noted that edge effects apparent in parallel plate capacitors can be reduced and capacitance, potential across plates and electric field between plates may be increased if the plates are identically dished in shape and separated by a layer of dielectric material of variable width, thickest adjacent to the edges and in the region of greatest curvature and gradually tapered to a constant lesser thickness in the region in between. These teachings have been useful in providing high capacitance devices to the electric-arts, but the geometries of such devices are such that the regions of substantially uniform electric field are not suitable for protecting components both because they are extremely thin and because the fields would not remain uniform under surge conditions.
Similarly, A. N. Hovland in U.S. Pat. No. 1,139,976, has shown that capacitance may be increased by extending the layer of dielectric separating the conductive members of a capacitor beyond the edges of the conductive members of increase flashover distance, and that such devices may be made in the form of truncated cones and stacked if space is a problem. The hollow space created between two stacked Hovland devices is surrounded by conductive materials which is generally at the same potential level and thus has a relatively small field. The space is not isolated from the effects of transient induced surges however, and accordingly is not suitable for electrical circuit component protection.